This invention relates to printing and copying devices and more particularly to such devices wherein recording is accomplished by means of a noncontact jet drop print head. The general principles of ink jet printing are illustrated by the type of recorders taught by Beam, U.S. Pat. No. 3,577,198, and Mathis, U.S. Pat. No. 3,701,998. Such jet drop recorders comprise a series of electrical and fluidic components, including an orifice plate and a charge plate arranged in laminar relationship for generating one or more rows of jets and selectively charging drops originating therefrom. Typically there may be about 250 or more jets formed in each such row, and each jet is stimulated to produce drops at the rate of about 40 Kilohertz.
All such drops fall through an electrical deflection field, and those which are charged are deflected into a catcher device. The uncharged drops, after passing through the field, are deposited on a print medium. Depending on the particular printing configuration utilized, the print medium may be a moving web of paper, a sheet of paper supported on a rotary drum, or a sheet of paper supported on an endless conveyor belt. For example, the above mentioned Mathis patent prints on a moving web transported below the printing head. Behane et al, U.S. Pat. No. 3,604,846, Loughren, U.S. Pat. No. Re. 27,555, and Fox et al, U.S. Pat. No. 4,063,254, all print onto a sheet of paper supported on a rotary drum. An endless conveyor belt printing support device is shown by Gamblin and Marinoff in a commonly assigned application entitled "RECIPROCATING PAPER HANDLING APPARATUS FOR USE IN AN INK JET COPIER" filed on even date herewith.
However, no matter what printing configuration is utilized, one of the most difficult problems in the operation of such jet drop recording devices is that of achieving satisfactory startup. When ink of other recording liquid is pumped into an initially empty recording head, the jets do not begin to flow freely but rather tend to blob and run together. This wets the top surface of the charge plate, including electrical lead lines plated thereon and ring-type charging electrodes connected thereto. Once such wetting has occurred, it is impossible to perform satisfactory drop charging, and any attempt at normal operation can result in shorting out of the lead lines and the charging electrodes. Also, liquid on top of the charge plate may affect jet straightness. Thus, in early designs of jet drop recording apparatus it was necessary to follow the startup step with a difficult cleaning step which was conducted while the jets were running. Not until after this cleanup operation was the apparatus ready for recording.
Numerous attempts have been made to achieve satisfactory startup. Martinez et al, U.S. Pat. No. 3,661,304, teaches use of a shock wave to force ink through a jet forming orifice at high energy. This method avoids blobbing at the exit side of the orifice and is fairly satisfactory for starting up a single jet. However, the technique has not been found to be satisfactory in starting up the large numbers of jets required for high speed printing and copying.
Satisfactory startup methods for large numbers of jets are disclosed in Stoneburner, U.S. Pat. No. 3,891,121, and Duffield, U.S. Pat. No. 3,970,222. In those methods, a source of pressurized air is connected to the recording head, and high pressure air is pumped into the head prior to the admission of ink. Once a flow of pressurized air has been established through the jet forming orifices, ink is pumped into the recording head. When the ink follows the pressurized air in this manner, startup is facilitated without any wetting of the charging electrodes.
As further taught by Stoneburner, startup may be improved by pumping a flush fluid through the recording head after the flow of pressurized air has commenced and before admission of ink. The flush fluid forms free flowing jets, and after these jets have been established, the flush fluid is replaced with ink.
Other startup methods involve moving the charge plate or the entire printing head assembly out of their normal positions during startup. Paranjpe, U.S. Pat. No. 4,031,561, teaches placing a movable charge plate in close contact with an orifice plate during startup operations of an ink jet recorder; after startup, the charge plate is returned to its normal position. Van Breemen et al, U.S. Pat. No. 4,081,804, teaches retracting the print web transport mechanism in an ink jet recorder during startup and replacing it with a catch pan until ink jet flow has been stabilized and adjusted. Chen et al, U.S. Pat. No. 3,839,721, shows a vapor chamber which is moved into position as the charge electrodes are withdrawn to prevent collection of dried ink and spraying during shutdown of an ink jet recorder.
Still other startup methods have displaced the printing head assembly from its operation position with respect to a printing web so that a catch trough can be positioned between the printing head and web. However, the movement of the printing head tends to dislodge accumulated particulate matter previously deposited in the printing head and associated fluid lines and valves so that during startup this dislodged particulate matter may plug the jet orifices and render them inoperative.
Accordingly, the need still exists in the art for providing a startup assist procedure which avoids mechanical displacement of an ink jet printing head assembly and its resultant problems, and at the same time provides a convenient mode of disposal for the flushing fluid and excess ink expelled from the orifices during startup and prior to printing.